The present invention is directed to an FM broadcasting system in which an auxiliary audio program and a data signal are transmitted on a subcarrier of an FM transmission channel.
A number of systems have been developed for transmitting auxiliary programs together with the main program being broadcast on an FM transmission channel. Commercial FM broadcast stations can now broadcast a number of programs in addition to the main program including FM stereo multiplex or other programs transmitted in the SCA (Subsidiary Communications Authorization) band. These commercial FM broadcast stations normally transmit the main program on the transmission channel carrier and provide background music or other auxiliary programs for subscribers in the SCA bands.
Examples of broadcasting systems in which the subcarriers of an FM transmission channel are used to transmit auxiliary programs can be found in the patent literature. In U.S. Pat. No. 3,122,610, an FM stereo multiplex transmission system is shown in which an auxiliary program, such as background music, is transmitted on a 67 KHz subcarrier with a frequency deviation of .+-.8 KHz. Thus, since the SCA band is defined as a frequency band in the range 58-75 KHz (centered at 67 KHz), this patent shows the use of the SCA band for the transmission of an auxiliary audio program. Similar uses of an FM transmission channel are described in U.S. Pat. No. 3,317,883; U.S. Pat. No. 3,534,266; and U.S. Pat. No. 3,980,954. Auxiliary programs also can be transmitted in available frequency bands in a similar manner in an AM broadcasting system as shown in U.S. Pat. Nos. 3,160,812 and 3,714,375; a microwave communication system as shown in U.S. Pat. No. 3,773,979; and a TV video system as shown in U.S. Pat. No. 3,842,196. Thus, different techniques are known in the art for transmitting auxiliary programs over subcarriers in various transmission systems.
In some prior art FM broadcasting systems, the auxiliary program includes an audio signal as well as other information which also can be transmitted on the subcarriers of the FM transmission channel. For example, it is known in the prior art to interrupt an auxiliary program such as background music with commercial messages intended for selected customers. The commercial messages usually are preceded or accompanied by an address code which designates the selected customer. One such system is disclosed in U.S. Pat. No. 3,922,607 in which an auxiliary audio program (background music) is transmitted on one subcarrier in the SCA band and a plurality of sequential messages, accompanied by coded signals, are transmitted on a second subcarrier. The sequential messages only interrupt the background music at the receiver designated by the coded signals. A similar system is disclosed in U.S. Pat. No. 3,714,575 in which the background music is periodically interrupted by speech signals. In both these patents, different subcarriers are used for the background music and the other information signals. However, the subcarrier having the information signals includes both the message/speech signals and the code signals on a time share basis.
Although the above prior art broadcasting systems provide a more efficient use of the FM frequency band, none of these prior art systems provides an efficient method for transmitting a data signal, such as digital information, on the SCA band in addition to an auxiliary audio program, such as background music. Time sharing broadcasting systems are impracticed for transmission of both background music and digital information because an interruption occurs in the transmission of the background music. Alternatively, the SCA band in an FM broadcasting system could be divided between an auxiliary audio signal and a data signal. However, by dividing the SCA band, a narrow bandwidth results for both the auxiliary audio program and the data signal. Therefore, even though large mass users have a need for both an auxiliary audio program and digital information, it has not been practical to transmit a data signal such as digital information and an auxiliary audio program on the same subcarriers in the SCA band in an FM broadcasting system.